Mass transfer apparatus

ABSTRACT

Mass transfer apparatus such as a distillation or rectification column which is divided so that the vapor flow is in two streams each in a separate zone, and wherein in each zone the liquid/vapor contacting stages have an inlet and an outlet disposed on opposite sides of an active area but to the same side of the division and such that liquid flow from the inlet across the active area to the outlet is generally rectiliner and parallel to the plane of the division, the locations of the stages in one zone being staggered axially relative to those of the stages in the other zone along the column and the liquid from each outlet being directed to an inlet of the next lower stage in the column whereby the liquid flow down the column is directed alternately from one zone to the other as it is passed successively from stage to stage down the column. This arrangement increases the number of theoretical liquid/vapor contacting stages that can be provided in a column of given length and maximizes mass transfer point efficiency enhancement with an uncomplicated design which permits ready modification of existing columns.

This is a divisional application of copending application Ser. No.338,541, filed Jan. 11, 1982 now U.S. Pat. No. 4,496,430.

This invention relates to a mass transfer apparatus, e.g. distillationcolumn or absorption column, of the kind having a plurality ofliquid/vapour contacting stages spaced apart one above the other in atower or column. Each stage has an active area where intimate contact iseffected between a rising vapour stream and a liquid stream flowing overthe stage as it travels from stage to stage down the column, whereby toeffect exchange of components between the liquid and vapour streams.

It has been proposed to divide the column into two or more verticalzones so that the vapour flow up the column is divided into at least twoseparate streams, and to divide the liquid/vapour contacting stages,e.g. bubble cap trays, into segments, one for each of the verticalzones, the segments being offset vertically relative to each other andthe arrangement being such that the liquid stream descending the columnis passed downwards in a stepwise manner from segment to segment andtravels through each zone in sequence as it flows down the column. Insuch an arrangement having n zones, the descending liquid stream issubjected to n times as many contacting stages as each of the n vapourstreams ascending the column and thus can make n times as many contactswith different vapour compositions as is the case in a column ofconventional design with the same axial spacing between liquid/vapourcontacting stages as between adjacent stages in one of the zones of theapparatus. Such a construction therefore permits a significant increasein the number of theoretical liquid/vapour contacting stages with acolumn of given length.

It has also been recognised that as the downflowing liquid stream flowsacross each tray there may be a tendency for the ratio of high boilingcomponents to low boiling components in its composition to increase,and, if so, improved mass transfer point efficiency enhancement isfavoured if the liquid is caused to flow in the same direction acrosseach tray so that the trend of enrichment of high boiling components inthe liquid is in the same direction across each tray.

However, the specific arrangements of mass transfer apparatus that havebeen proposed hitherto to achieve these effects have suffered from oneor more of the following defects.

The relative locations of inlet and outlet for each tray have been suchas to provide a "short circuit" in the liquid flow across the tray andthus cause substantial under-utilisation of the available tray.

The liquid flow pattern causes variation in the degree of submergenceover the area of each tray thereby reducing mass transfer pointefficiency.

The liquid flow pattern causes different flow path lengths and henceliquid residence times and/or induces substantial back- or side-mixingon each tray thereby reducing or negating any potential benefits ofhaving the liquid pass in the same direction across each tray because ofthe concentration gradient effect.

The arrangement requires the liquid leaving one tray to be transferredfrom one side of the column to the other to be fed to the inlet to thenext tray, thus necessitating the provision of transverse pipes withconcomitant complication of column design, increase of expense andlimitation in the size of the outlet which in turn severely limits theliquid load that can be carried by the column. The width of the outletfrom each tray is limited to not more than one half of the diameter ofthe column, thereby severely limiting the maximum liquid load relativeto vapour that can be handled by the column.

The present invention provides a design of mass transfer apparatus ofthe kind having two vertical zones and offset of the liquid/vapourcontacting stages in one zone relative to the other, but in which theabove-mentioned disadvantages are reduced or avoided.

According to the present invention, there is provided mass transferapparatus including a generally vertical columnar section having anaxially extending divider which divides the section into two parallelzones so that vapour flow up the section is in two separate streams;each zone having a plurality of liquid/vapour contacting stages locatedat axially spaced intervals therealong, each stage having disposed toone side of an active area thereof an inlet for the supply of liquid tosaid area and on the opposite side of that area an outlet for drainingliquid from said area, said inlet and outlet being disposed to the sameside of the divider as the liquid/vapour contacting stage and such thatliquid flow from the inlet across the active area to the outlet isgenerally rectilinear and parallel to the divider, the locations of thestages in one zone being staggered axially relative to those of thestages in the other zone along the columnar section; and each outletbeing connected by a downcomer for liquid flow therethrough to an inletof the next lower stage in the section whereby the liquid flow down thesection is directed alternately from one zone to the other as it ispassed successively from stage to stage down the section.

The invention also provides an assemblage of parts includingliquid/vapour contacting stages and/or sections thereof, said assemblagebeing adapted for assembly within a hollow vertical columnar casing toform said mass transfer apparatus.

In the apparatus of the invention, the descending liquid stream issubjected to twice as many vapour-liquid contacting stages as each ofthe two vapour streams ascending the column and thus can make twice asmany contacts with different vapour compositions as would be the case ina column of conventional design with the same axial spacing betweenstages as between the stages in either zone of the apparatus of theinvention. Accordingly, an increase in the number of effectiveequilibrium mass-transfer stages in a given length of column is achievedand thereby an enhanced degree of separation for the same energyconsumption. Alternatively, the same degree of separation as aconventional column can be achieved with the expenditure of less energy.Conversely, for the same energy consumption, the same degree offractionation can be achieved with a lesser number of actualliquid/vapour contacting stages, thus reducing the capital cost of thecolumn and internals.

Further, in each zone of the apparatus of the invention, the downflowingliquid flows in the same direction across each active area in all theliquid/vapour contacting stages in that zone. Thus the trend ofenrichment of higher boiling components in the liquid is in the samedirection across each stage in the zone, thus providing the idealarrangement for mass transfer point efficiency enhancement byconcentration gradient.

Arranging for the inlet and outlet areas associated with an active areaof each liquid/vapour contacting stage to be disposed on opposite sidesof the active area such that the liquid flow across the active area isgenerally rectilinear suppresses tendencies for side- or back-mixing,thereby improving the mass transfer point efficiency enhancementrelative to overall efficiency that is achievable from causing theliquid to flow in the same direction across each liquid/vapourcontacting stage in a zone of the column.

This arrangement also permits the inlet and outlet to extend in adirection generally transverse, e.g. generally normal, to the plane ofthe divider. In one convenient embodiment, each liquid/vapour contactingstage is in effect divided into an inlet area, active area and outletarea by notional dividing lines each extending at an angle, andpreferably normal, to the plane of the divider.

By an inlet area we mean an area of the stage which receives liquid froma preceding stage for delivery through the inlet to an active area, andby an outlet area, we mean an area into which liquid from an active areadrains from an outlet. In general, an inlet area will correspond to thatarea of a stage which is under a downcomer and an outlet area willcorrespond to or define the mouth of a downcomer, and the inlet andoutlet will, in this embodiment, lie generally in the plane dividing theinlet area and outlet area, respectively, from the active area.

In the above embodiment, where each stage has a single active area themaximum inlet or outlet width that is practicable is about 40% of thecolumn diameter and thus it is most suitable for cases where the vapourload is expected to be substantially the dominant load. However thearrangement also permits the ready division of each liquid/vapourcontacting stage into a plurality of active areas each lying between aninlet area which receives liquid draining from an active area of thepreceding stage in the section and an outlet for draining liquid from anactive area beside it. By means of such division, the effective totalinlet and outlet width of each stage can be substantially increased,e.g. up to as much as 1.8 times the column diameter, thereby providingthe capability for coping with higher liquid/vapour load ratios.

Where a liquid/vapour contacting stage has two or more active areas eachlying between an inlet area and an outlet area, the effective length ofthe outermost inlet and/or outlet, which will be located at the positionhaving the smallest chordal length of all the inlets and outlets of thatstage, may be increased, if desired, by making it curved, e.g. to runparallel to the wall of the column or angled e.g. cranked ordouble-cranked.

The locations, across the width of the divider, of the active areas ofthe stages in one zone can correspond with those of the stages in theother zone and the dispositions of the inlet and outlet areas of thestages in one zone can correspond with those of the outlet and inletareas, respectively, of the stages in the other zone, and causing liquidflow across each active area of each liquid/vapour contacting stage ineach of the zones to be in the same direction as across each of thecorresponding active areas above and below it, without the need toprovide tranversely directed transfer pipes.

One preferred arrangement providing a plurality of active areas on eachstage, and permitting this preferred disposition of active areas, inletareas and outlet areas in each zone, is for each stage to have 2n activeareas, p inlet areas and q outlet areas, where n is a positive integerand where in one of the zones p=n and q=n+1 and in the other of thezones p=n+1 and q=n, with each liquid/vapour contacting stage being, ineffect, divided into said active areas, inlet areas and outlet areas bynotional dividing lines extending in a direction generally normal to theplane of said vertically extending division.

In general, n will be 1 or 2. Where n is 1, so that each liquid/vapourcontacting stage has two active areas, one of the zones includes stageshaving an inlet area, an active area disposed to each side of said inletarea and an outlet area disposed to the other side of each active areafrom the inlet area, and the other of the zones includes stages havingan outlet area, an active area disposed to each side of said outlet areaand an inlet area disposed to the other side of each active area fromthe outlet area.

In the above arrangement, each stage will have an even number of activeareas. However, it is also possible for each stage to have an odd numberof active areas and still obtain the above-mentioned preferreddisposition of active areas, inlet areas and outlet areas.

An important practical aspect of the present invention is that itsarrangement is such that the internal constructions of existing single-and multi-pass mass transfer apparatus can readily be modified to be inaccordance therewith.

It will be understood that for most applications the divider will belocated generally diametrically so as to divide the section into twozones of equal or substantially equal areas. It will also be usual forthe liquid/vapour contacting stages in each of the zones to be spaced atgenerally equal axial intervals along the zone and for the axiallocations of the stages in one zone to be staggered by a distance x/2relative to the axial locations of the stages in the other zone, where xis the distance between adjacent stages in one of the zones.

The active areas of the liquid/vapour contacting stages may be formed inconventional fashion, e.g. using sieve tray-, valve tray- and bubble captray-sections.

The internal constructions of the apparatus according to the inventionmay be formed from a plurality of individual plates and othercomponents. The plates and other components may be sized so as to becapable of being introduced into preformed columns or towers throughexisting ports or manholes and may be designed to be fixed togetherwithin the tower or column in conventional manner, e.g. using nuts andbolts, and attached to the column wall using conventional or existingfixing means, e.g. bolting bars and peripheral rings, therebyfacilitating the ready modification of existing mass transfer apparatusto operate according to the invention.

The invention is now described in greater detail with reference to theaccompanying drawings wherein

FIG. 1 is a vertical cross-sectional view in diagrammatic form throughpart of the length of a single pass mass transfer column of conventionaldesign, e.g. a distillation column, omitting the reboil, reflux and feedsections, and showing a few liquid/vapour contacting stages or trays;

FIGS. 2 and 3 are cross-sectional plan views of the apparatus of FIG. 1at XX' and YY', respectively;

FIG. 4 illustrates the principle of operation of a single-pass masstransfer apparatus of FIGS. 1, 2 and 3 when modified according to thepresent invention, and viewed at right angles to the view of theconventional column shown in FIG. 1;

FIG. 5 is an isometric view showing one internal constructionarrangement for a mass transfer column operating in accordance with theprinciple illustrated in FIG. 4;

FIG. 6 is a cross-sectional plan view of the arrangement of FIG. 5,located in a column;

FIG. 7 is a part isometric cutaway view showing the internalconstruction of part of a column in accordance with another embodimentof the invention, and which is, in effect, a modified two-pass column;

FIG. 8 is a diametrically opposed view of the construction illustratedin FIG. 5;

FIGS. 9 and 10 are cross-sectional plan representations of the column ofFIGS. 7 and 8 at AA' and BB', respectively, of FIG. 7;

FIGS. 11 and 12 illustrate modifications of the embodiment of FIGS. 7 to10 to increase the effective weir or outlet width of each half tray andcorrespond to the cross-section shown in FIG. 9;

FIGS. 13 and 14 are cross-sectional plan representations taken at levelscorresponding to those of FIGS. 9 and 10 of a column according to theinvention which is in effect a modified four-pass column; and

FIGS. 15 and 16 are cross-sectional plan representations, taken atlevels corresponding to those of FIGS. 9 and 10, of a column accordingto the invention which is in effect a modified three-pass column.

A conventional single pass mass transfer, e.g. distillation, column suchas illustrated in FIGS. 1, 2 and 3 includes a plurality of liquid/vapourcontacting stages, A, B, C, D, E, etc, located within a generallyvertical casing 2. The liquid/vapour contacting stages may take the formof sieve trays, valve trays, bubble cap trays or any other known form ofconstruction permitting rising vapour to pass through and intimatelycontact liquid over the active area of each stage. Associated with eachstage is a downcomer 4A, 4B, etc, the arrangement being such that liquidsupplied to each tray flows across the active area of the tray to drain(e.g. over an outlet weir such as 6A, 6B, etc) into a downcomer by meansof which it is supplied to the inlet area 8B, 8C, etc to the next traybelow in the column across the active area of which it flows to the weirand downcomer associated with that next tray, and so on in a zig-zagfashion down the tower while the vapour passes up the tower through theactive area of each tray in well known fashion. As seen best in FIGS. 2and 3, the downcomer for liquid flowing from alternate trays A, C, E,etc is generally a vertical channel whose cross section is defined by anarc 10 of the column and a chord 12 bisecting the arc and the inlet areafor those trays is of corresponding cross-section but in a diametricallyopposed location, bounded by arc 10' and chord 12', and the locations ofthe downcomer and inlet area for the trays B, D, etc are reversedwhereby the downcomer from one plate supplies the inlet area to thenext.

The separation that can be achieved by the column depends inter alia onthe reflux ratio employed and the number of contacting stages (or trays)provided in the column and of course increasing the possible separationby increasing the number of trays generally increases the height of thecolumn. If the trays are located too close to each other then dependingon the liquid/vapour ratio in the distillation zone there is a tendencyfor downcomer flooding to occur or for liquid entrained in vapourpassing through one tray being carried up into contact with the liquidin the next higher tray (so-called "entrainment flooding"), thusinterfering with the separation effect.

Referring to FIG. 4 of the drawings, in its simplest form the apparatusof the present invention may be regarded, in effect, as a modificationof the conventional single pass mass transfer apparatus illustrated inFIGS. 1, 2 and 3. The column 2 is divided into two axially extendingparallel zones 14, 16 by a diametric divider plate 18 which extendsaxially in a vertical plane which is generally normal to the chords 10,12, 10', 12', and bisects the vertical planes in which said chords lie.Thus each liquid/vapour contacting stage A, B, C, etc is, in effect,divided into two halves 14A and 16A, 14B and 16B, 14C and 16C etc, withone half in each of the zones. The axial locations of the two halves ofeach stage are staggered with respect to each other and in theembodiment illustrated the spacing between the two halves, which is thepreferred spacing for optimum operation, is x/2 where x is the axialdistance between adjacent trays in the conventional arrangementillustrated in FIGS. 1, 2 and 3.

As illustrated by the solid flow lines in FIG. 4, the internalarrangement of the column is so designed and constructed that the liquiddraining from each half tray is directed through the divider 18 to thenext lower half tray which is in the other zone of the column. Thus, theliquid is directed from one zone to the other as it flows from half-trayto half-tray down the column. The vapour flow up the column illustratedby the broken lines is divided into two separate streams by the divider.Thus, the liquid flowing down the column contacts each vapour streamalternately and the descending liquid stream is subjected to twice asmany vapour-liquid contacting stages as the vapour ascending the column.

In the disposition of the column as illustrated in FIG. 4, the flow ofthe liquid across each tray is in a direction normal to the plane of thedrawing and is reversed as it flows from one zone to the other. Thus,for example, for half trays 14A, 14B, etc, the flow may be towards theviewer out of the plane of the drawing and for the other half trays,16A, 16B, etc, it will be away from the viewer, into the plane of thedrawing.

One suitable internal arrangement for a column operating in accordancewith the principle illustrated in FIG. 4 is illustrated in FIGS. 5 and 6of the drawings.

Referring to FIG. 5, which is an isometric, part cutaway view of thearrangement showing two liquid/vapour contacting stages only, thedivider is provided by a generally diametrically disposed axiallyextending baffle plate 20 and each of the two zones 14, 16 is providedwith a plurality of half trays spaced vertically one above the other,two of which, 14B and 16B, one in each zone, are illustrated. Each halftray is generally semicircular in plan but with one extremity,comprising a segmental portion lying between the inner wall 21 (FIG. 6)of the column and a vertical plane extending normally from the plane ofthe baffle plate 20 and located towards one end of the baffle plate, cutaway to provide the outlet area connecting to a downcomer. The otherextremity, comprising a segmental portion lying between the wall 21 ofthe column and a second vertical plane extending normally from the planeof the baffle plate 20 and located towards the opposite end of thebaffle plate to that of said first vertical plane, forms the inlet areato the half tray. The remaining, central, portion is available to formthe active area of the half tray.

For the half trays in zone 14, i.e. tray 14B in the drawing, the inletand outlet areas are identified as 24 and 22, respectively and for halftrays in zone 16, i.e. tray 16B in the drawing, the correspondingfeatures are identified as 24' and 22', respectively. (FIG. 5) Therespective active areas are identified as 26 and 26'. The inlet andoutlet areas for the half trays in one zone are located at opposite endsof the dividing plate to those in the other zone.

Liquid entering each outlet area 22 is passed to the inlet area 24' ofthe next lower half tray in the column by a downcomer and likewise foreach outlet area 24 and inlet area 22'. The downcomer from each outletarea 22 comprises a vertical channel lying between the inner wall 21 ofthe column and a vertical plate 28 which extends between the half trayin zone 14, e.g. 14B, and the next lower half tray in zone 16 in thecolumn, e.g. 16B, in the vertical plane defining the lip (outlet) of theoutlet area 22 and across the full width of the column in that plane.

So that the downcomer may feed to the inlet area 24' of the next lowerhalf tray down the column, a gap is provided at the bottom of that halfof the plate 28 which extends into zone 16, to provide a mouth (inlet)30' and the end portion of the baffle plate 20 extending outwardly fromplate 28 is omitted.

To prevent vapour from zone 16 entering zone 14, a head plate 32 sealsthe top of the downcomer space between plate 28 and the inner wall ofthe column in zone 16 and the bottom of the downcomer space in zone 14is sealed by an extension 34' of plate 24'.

To provide a downcomer to supply liquid entering each outlet area 22' tothe inlet area 24 of the next lower half tray a corresponding plate 28'is provided which extends between each half tray in zone 16 (e.g. 16A,not shown) and the next lower half tray in zone 14, e.g. 14B, the platehaving a gap to provide mouth (inlet) 30'. The downcomer has a headplate 32' corresponding to plate 32 and the bottom of the downcomerspace in zone 16 is sealed by an extension 34 of plate 24, correspondingto 34' of plate 24'.

Although only two half plates and associated baffle and downcomerarrangements are shown, it will be understood that the arrangementillustrated may be repeated one or more times along the length of thecolumn.

The depth of fluid on each half tray is controlled by a weir 36, 36' andto prevent vapour travelling up the downcomers, the height of the weirshould be such that the inlet 30, 30' of each tray is submerged.

In operation, liquid provided to an inlet area 24 passes through mouth(inlet) 30, travels across the active area 26 of a tray 14A, 14B etc inzone 14 of the column and passes into outlet area 22 from which area itis directed through the downcomer to the mouth (inlet) 30' of the inletarea 24' to an active area 26' of the next lower half plate in thecolumn in zone 16 and flows across that active area to the outlet area22' whence it is directed by the next downcomer to the inlet area 24 ofthe next lower half tray in the column in zone 14, and so on down thecolumn. Thus, as the liquid flow travels down the column it swaps fromzone to zone as it travels from half-tray to half-tray.

Vapour released from the liquid travels up the column in two separatestreams, one in zone 14 and the other in zone 16. Thus, vapour flowingupwardly in zone 14 contacts only trays 14A, 14B, etc, but in reverseorder, and vapour flowing upwardly in zone 16 contacts only trays 16A,16B, etc, but in reverse order. On the other hand, the downward flowingliquid is in contact alternately with vapour in zone 14 and then vapourin zone 16 as it passes from tray 14A to tray 16A to tray 14B to tray16B and so on down the column.

It will thus be seen that by means of the arrangement according to theinvention, whereas the vapour stream rising up each of zones 14 and 16will make the same number of contacts with the downflowing liquid as ina column of conventional design with the same axial spacing betweentrays, the liquid will make twice as many contacts with different vapourcompositions than would be the case in a column of conventional designwith the same axial tray spacing. Also, whereas in the conventionalcolumn illustrated in FIGS. 1, 2 and 3, the liquid on any tray iscontacted by vapour from the next lower liquid/vapour contacting stagein the column, in the arrangement according to the invention the liquidon that tray is contacted by vapour from the next-but-one lowervapour/liquid contacting stage in the column. Thus, the inventionprovides an increase in the number of effective equilibriummass-transfer stages within a given length of column and thereby anenhanced degree of separation achievable for the same energyconsumption. Alternatively the same degree of separation as in asimilarly sized conventional mass-transfer column can be achieved at theexpenditure of less energy since a smaller reflux ratio, and hence asmaller amount of reboil, will be required. Additionally, forvapour-liquid ratios for which this arrangement is most suitable, namelywhere the vapour load tends to predominate, it is possible to handlegreater loads within a given diameter column than in the case of aconventional column.

A further advantage of a mass transfer column having the arrangement ofthe present invention is that, especially for distillations wherein thevapour/liquid ratio is high, its ultimate load handling capacity for agiven efficiency is greater than that achievable in a conventionalcolumn such as illustrated in FIGS. 1, 2 and 3. The arrangement isparticularly suitable for handling high vapour/liquid ratios because theeffective tray spacing in respect of entrainment flood, where liquid onone tray is carried by the vapour stream up to contact in the next trayabove it in the column, is double the effective tray spacing in respectof liquid downcomer flood.

It will also be seen that in the apparatus according to the inventionthe liquid flow across all the half trays in a zone is always in thesame direction, thereby providing a greater enhancement of mass transferpoint efficiency to overall efficiency as compared with a conventionalcolumn where the liquid flow down the column is reversed from tray totray. This is because as the liquid flows across each tray in a masstransfer apparatus, there is a general trend of enrichment in higherboiling components in the liquid. In the arrangement of the presentinvention, the vapour rising in each zone from the enriched liquid inone tray contacts the enriched liquid in the next tray above in thatzone and the vapour rising from the lean liquid contacts the lean liquidin the next tray above whereas in a conventional column the vapourrising from the enriched liquid contacts the lean liquid in the nexttray above and the vapour rising from the lean liquid contacts theenriched liquid in the next tray above.

As the liquid descends from half tray to half tray, its direction oftravel is reversed and the flow is generally back-and-forth, or zig-zag.This promotes uniformity of liquid distribution over the active area 26,26' of each half tray, as compared with an arrangement where the liquidflow is more generally arcuate, and reduces any tendency to variation ofdegree of submergence over the active area due to swirling. The formerimproves effective utilisation of the active area of the half tray andthus further enhances mass transfer efficiency and the latter improvespoint efficiency.

On each half tray, the liquid flow from inlet to outlet is generallyrectilinear, thereby suppressing tendencies to back- and side-mixing andaccordingly maximising the mass transfer efficiency enhancementobtainable by achieving liquid flow in the same direction on allhalf-trays in each zone of the column. The flow paths across each halftray are also all of substantially equal path length, thereby ensuringuniform residence times and there is no possibility of a shortcircuit ofthe liquid flow.

The same liquid flow direction across each half tray in each zone isalso achieved without the need to provide a transversely mountedtransfer pipe to direct the liquid from one side of the column to theother between trays.

In the arrangement of FIGS. 5 and 6, the effective width Z (FIG. 6) ofthe outlet or weir 36, 36' associated with each tray is limited. Theeffective width can only be increased by reducing the active area of thehalf tray. However, if each liquid/vapour contacting stage has aplurality of active areas a substantial increase in effective outlet orweir width can be obtained without sacrificing any of the otheradvantages of the invention.

One form of construction for such an arrangement, which is in effect amodification of a conventional two pass system, is illustrated in FIGS.7 to 10 in which reference numeral 102 represents the wall of the columnand reference numeral 104 is an axially extending diametrically disposedplate which in effect divides the tray section of the column into twoliquid/vapour contacting zones 106, 108 each of which is provided with aseries of trays 110A, 110B, 110C etc, and 112A, 112B, 112C etc,respectively, of which only three are shown for convenience. As in thearrangement illustrated in FIGS. 5 and 6, the axial locations of thehalf-trays in zone 106 are staggered with respect to those of thehalf-trays in zone 108 whereby the axial distance between adjacent halftrays in the column is x/2 where x is the axial distance betweenadjacent half trays in either of the zones 106, 108 of the column.

In this embodiment, each half tray comprises two active, orliquid/vapour contacting areas formed in effect from a semicircle fromwhich has been omitted (a) a central portion (at the widest part of thesemicircle) defined as lying between two major chords extending parallelto each other and at right angles to the plate 104 and from the plate104 to the circumference of the semicircle and (b) two end portions eachcomprising the portion of the semicircle lying outside a further minorchord extending at right angles from the plate 104 to the circumference.Thus, the two active, or liquid/vapour contacting areas of each halftray comprise two truncated quadrant sections identified as X and X' forhalf trays in zone 106 and as Y and Y' for half trays in zone 108.

For the half trays 110A, 110B, etc in zone 106, the said central portionforms the base plate 114 of a downcomer through which liquid is suppliedto the tray and comprises an inlet area for supplying liquid to theactive areas X and X'. The two end portions form the outlet areas 116for each of the contacting areas and each connects with a downcomer forliquid draining from the half tray.

For the half trays 112A, 112B, etc in zone 108, the said central portionis absent, providing a gap 118 for liquid draining from the half trayand comprises a common outlet area for the two liquid/vapour contactingareas Y and Y', and each of the two outer portions forms a base plate120 of a downcomer through which liquid is supplied to the half tray andcomprises an inlet area for supplying liquid to the areas Y and Y'.

Thus, the half trays 110A, 110B, etc in zone 106 are supplied withliquid to a centrally disposed inlet area and the liquid divides intotwo streams which flow outwardly over the liquid/vapour contacting areasX, X' to drain into downcomers from outlet areas disposed at the twoextremities of the half tray, and the half trays 112A, 112B, etc in zone108 are each supplied with liquid from two inlet areas supplied fromthese second-mentioned downcomers and the liquid from each said inletareas flows inwardly over a liquid/vapour contacting area Y or Y' todrain from a a centrally located outlet area which supplies the liquidvia a downcomer to the centrally disposed inlet area of the next halftray down in zone 106, and so on.

In the embodiment illustrated, the downcomers collecting liquid fromhalf trays in zone 106 and delivering it to half trays in zone 108 areformed by providing a pair of chordal plates 122, extending verticallybetween alternate pairs of half trays in the column; i.e., for example110A and 112A, 110B and 112B (as shown) etc, the plates extending fullyacross the chordal width of the column; i.e. extending from both sidesof the main column dividing plate 104. The inactive half at the top ofthese downcomers is sealed by a head plate 144 (see below).

At the level of the half tray 112 next below, a horizontal slit 124 isformed at the bottom end of each plate, on the other side of the planeof the main column dividing plate 104 from the outlet area 116, toprovide the inlet to the half tray 112 and the space between the chordalplate and the wall of the column on the other side of the plane of theplate 104 at the level of the half tray 112 is sealed off by horizontalbase plate 126.

To permit liquid draining from outlet area 116 to pass from zone 106 tozone 108, the portion of the main dividing plate extending between eachchordal plate 122 and the column wall 102 and for the vertical distanceextending downwards from a half tray 110 to the next half tray 112 therebelow is omitted (see FIGS. 7 and 8).

Thus, liquid flowing into outlet area 116 is caught on plate 126 andflows over base plate 120 and through slit inlet 124 on to a section Yor Y' of a half tray 112 across which it flows to outlet area ordowncomer opening 118.

To direct liquid from each outlet area 118 from a half tray 112 in zone108 to a central portion of the next half tray 110 below in the zone106, a second pair of chordal plates 130 are provided which extendvertically between the other pairs of adjacent trays, e.g. 112B and 110Cas shown, the plates extending across the full chordal width of thecolumn, i.e. extending from both sides of main column dividing plate104, and in the planes of the two opposed edges of the area 118. At thelevel of the lower half tray, i.e. a half tray 110 in zone 106, ahorizontal slit 132 is formed at the bottom of each plate on the otherside of the main column dividing plate from the outlet 118, to provideinlets to each section of a half tray 110, and the space between theplates 130 on the other side of main column dividing plate 104 at thelevel of half tray 110 is sealed by horizontal plate 134. To permitliquid draining through outlet area 118 to pass from zone 108 back tozone 106, the portion of the main dividing plate extending between thechordal plates 130 and for the vertical distance extending downwardsfrom half tray 112 to the half tray 110 next below, is omitted.

Thus, liquid flowing from a plate 112 through outlet area 118 is caughton plate 134 and flows over base plate 114 and through slit inlets 132on to the sections X, X' of a half tray 110 across which it flows toexits or drains 116.

The outlet areas 116, 118 are provided with weirs 138, 140 whose heightis such as to ensure that the slit inlets 132 and 124 are sealed by ahead of liquid during distillation.

The gap between the top edges of the chordal plates 130 on the otherside of main column dividing plate 104 from outlet areas 118 is sealedwith a head plate 142 to prevent ingress of vapour travelling up throughzone 106 and likewise the gaps between the top edges of chordal plates122 and the wall 102 of the column on the other side of the main columndividing plate 104 from outlet areas 116 are sealed with head plates 144to prevent ingress of vapour travelling up through zone 108.

To the same general benefits as are obtained by the embodiment of FIGS.4 to 6, the embodiment of FIGS. 7 to 10 adds the advantages that theeffective weir or outlet width for each half tray is the sum of thewidths of weirs 138 or 140, respectively, and is therefore substantiallygreater, thereby enabling the handling of higher liquid/vapour ratios.

If desired, the effective widths of outlet weirs 138 may be increased bymaking them curved, e.g. to follow the curvature of the column wall, asillustrated in FIG. 11, or angled, e.g. cranked as illustrated in FIG.12. FIGS. 11 and 12 are cross-sections taken at the same level as FIG.9.

FIGS. 13 and 14 are simplified diagrammatic cross-sectional plan viewsat two levels of a mass transfer apparatus having an arrangementcorresponding to that of FIGS. 7 to 10 but wherein each stage has fouractive areas.

FIG. 13 is a plan view at a level corresponding to that of FIG. 9 andFIG. 14 is a plan view at a level corresponding to that of FIG. 10.

As illustrated in FIG. 11, in one zone the active areas 202, 204, 206,208 of the half tray are supplied with liquid from inlet areas 210 and212 and drain into downcomers from outlet areas 214, 216 and 218. In theother zone (as illustrated in FIG. 14) the active areas 202', 204', 206'and 208' are supplied from inlet areas 214', 216' and 218' which receiveliquid from the downcomers fed from outlet areas 214, 216 and 218 of thenext higher stage in the other zone, and drain into downcomers fromoutlet areas 210' and 212' which feed inlet areas 210 and 212 of thenext lower stage in the other zone. 220 and 220' are head platescorresponding in function to the plates 142 and 144 in the arrangementof FIGS. 7 to 10.

The arrangements illustrated in FIGS. 7 to 10, and FIGS. 11 and 12 aremore complicated than that of FIGS. 4 to 6 but have the advantage ofbeing able to cope with substantially higher liquid loads because oftheir greater overall weir or outlet width per half tray.

FIGS. 15 and 16 are simplified diagrammatic cross-sectional plan viewsat the same levels as those of FIGS. 13 and 14, of a similar arrangementto that illustrated in FIGS. 13 and 14 but wherein each stage has threeactive areas. In one zone, as illustrated in FIG. 15, the active areas302, 304, 306 of each half tray are supplied with liquid from inletareas 310, 314 and drain into downcomers from outlet areas 308, 312. Inthe other zone, as illustrated in FIG. 16, the active areas 302', 304',306' are supplied with liquid from inlet areas 308', 312' and drain intodowncomers from outlet areas 310', 314'. 320 and 320' are head platescorresponding in function to the plates 142 and 144 in the arrangementof FIGS. 7 to 10.

It will be understood that for optimum performance of mass transfercolumns constructed in accordance with the invention, the effectiveareas of the liquid/vapour contacting stages in each zone should usuallybe essentially the same. However in a few cases this will not be so. Forexample where the liquid feed to the column is initially fed to one zoneonly and is heavily subcooled, a reduction of the vapour load in thatzone relative to the other zone will occur above the feedpoint and itmay be desirable to compensate for this by reducing the active areas ofthat zone relative to the other. Similarly, the axial distance betweenadjacent trays should also be the same in corresponding parts in all ofthe zones. Likewise, the spacing between axially adjacent liquid/vapourcontacting stages in any section of the column as a whole should beuniform; in other words, the spacing should be x/2 where x is the axialspacing between adjacent stages in either of the zones. However, thespacing between adjacent stages in one section of the column may differfrom that in another section of the column.

It is an important feature of the invention that the parallel axiallyextending liquid/vapour contacting zones are constructed and arrangedsuch that the upward vapour flows in each are kept essentially separate.However, in order to ensure pressure equalisation between the zones, itmay be desirable to provide for very limited transfer of vapour betweenzones at selected points up the column, e.g. by connecting the zoneswith small diameter pressure balance pipes through downcomers or byother means.

The whole or less than the whole of the section of a masstransfer columnhaving vapour/liquid contacting stages may be constructed in accordancewith the invention. For example, for some distillation applications itmay be desirable that only that portion above the feed point is soconstructed, with the portion below the feed point being of a differentconstruction, e.g. the form illustrated in FIG. 1.

Whereas in the drawings the means of preventing vapour passing from onezone to another comprise outlet weirs whose height ensures that theinlets to the trays are sealed with a head of liquid, it will beunderstood that alternative means may be used, such as by providinginlet weirs, in which case the outlet weirs may be omitted.

The invention is now further illustrated by the following Examples.

EXAMPLE 1

This example compares the fractionation achieved within an absorber ofconventional design and having four trays or stages operating at 100%efficiency, with that achieved by using an absorber having thearrangement illustrated in FIGS. 5 and 6 of the drawings, i.e. in whichin effect each of the 4 trays has been split into two whereby each ofthe two zones of the column has 4 stages each comprising a half tray,the spacings between adjacent trays in each of the zones being the sameas the spacing between the adjacent trays in the conventional column.The same feed components and the same operational efficiency areemployed in each column. In each case, the feed to the top tray was aliquid containing 99 moles per hour of pentane and 1 mole per hourhexane and the feed below the bottom tray was a vapour containing 50moles per hour pentane and 50 moles per hour hexane. In both cases, thetop feed was maintained at its bubble point temperature and the lowervapour feed was maintained at its dew point, to ensure a validcomparison of results, and the pressure in each case was maintained at30 psia.

The resultant top and bottom products recovered from the absorbers arecompared in the table below, while the improvement in fractionation isassessed in the form of a fractionation performance index. Fractionationperformance index is defined as moles per hour pentane (LK) in overheaddivided by moles per hour pentane (LK) in base multiplied by moles perhour hexane in base (HK) divided by moles per hour hexane (HK) inoverhead:

    ______________________________________                                                                           Fractionation                              Top        Lower   Top      Bottom Performance                                Feed       Feed    Product  Product                                                                              Index                                      ______________________________________                                        Conventional absorber                                                         Moles per                                                                             99.0   50.0    106.67 42.33  47.88                                    hour C.sub.5                                                                  Moles per                                                                              1.0   50.0     2.55  48.45                                           hour C.sub.6                                                                  Absorber of FIGS. 5 and 6                                                     Moles per                                                                             99.0   50.0    108.01 40.99  94.74                                    hour C.sub.5                                                                  Moles per                                                                              1.0   50.0     1.38  49.62                                           hour C.sub.6                                                                  ______________________________________                                    

It can easily be seen that modifying the absorber in accordance with theinvention has almost doubled the index of fractionation performance.

Moreover, enhanced operational efficiency is obtained from the secondabsorber due to the path followed by the descending liquid, the longerpath length of the liquid over each tray and the altered liquid tovapour mass ratio, compared to the conventional system.

EXAMPLES 2 TO 4 (Examples 2 and 4 are for the purposes of comparison)

In Examples 2 and 3 the fractionation achieved in a distillation columnof conventional design comprising three trays above the feed and threetrays below the feed (Example 2) is compared with the fractionationachieved in a column having the arrangement illustrated in FIGS. 7 to 10having 3 half trays in each of the two zones above the feed and the samenumber in each of the two zones below the feed (Example 3) and whereinin each zone the spacing between adjacent half trays is the same asbetween adjacent trays in the conventional column.

In each case 100% operation efficiency is taken for the trays.

In both cases the feed was 100 moles per hour pentane and 100 moles perhour hexane and the reflux ratio was maintained at 2.0. Reflux ratio isdefined as ratio of mass flow of reflux returned to the top tray dividedby the mass flow of distillate liquid product removed. Totalcondensation of tower overhead vapour was taken in both cases. The feedand reflux temperatures were maintained at their bubble pointsthroughout both Examples, and the pressure of operation similarlymaintained at 30 psia for both Examples, to ensure a valid comparison ofresults. The results are shown in the Table below. It will be seen thatthe present invention provides an improvement in fractionationperformance of 223%. Moreover enhanced operational efficiency isobtained in the process of Example 3 as compared with Example 2, due tothe path followed by the descending liquid, and the altered liquid tovapour mass ratios.

In Example 4, the distillation of Example 2 was repeated but with thereflux ratio raised to the level required to obtain the samefractionation performance index as that achieved in Example 3. Theresults are again shown in the Table from which it can be seen that anincrease in energy consumption of about 1.9×10⁶ ×BTU per hour isrequired; an increase of over 60%.

                                      TABLE                                       __________________________________________________________________________                                     Fractionation                                                                 Performance                                                 Top   Base  Reboiler                                                                            Index (calcu-                                     Feed  Reflux                                                                            Product                                                                             Product                                                                             Duty  lated as per                                 Example                                                                            (moles/hr)                                                                          Ratio                                                                             (moles/hr)                                                                          (moles/hr)                                                                          (mmbtu/hr)                                                                          Example 1)                                   __________________________________________________________________________    2    C.sub.5 100.0                                                                       2.00                                                                              89.272                                                                              10.728                                                                              3.08  66.01                                             C.sub.6 100.0                                                                           11.195                                                                              88.805                                                   3    C.sub.5 100.0                                                                       2.00                                                                              92.616                                                                               7.384                                                                              3.075 147.22                                            C.sub.6 100.0                                                                            7.851                                                                              92.149                                                   4    C.sub.5 100.0                                                                        3.893                                                                            92.619                                                                               7.381                                                                              4.968 147.3                                             C.sub.6 100.0                                                                            7.848                                                                              92.152                                                   __________________________________________________________________________

I claim:
 1. A structure for installation in a vertical column forforming a mass transfer apparatus, said structure comprising:plate meansadapted to be disposed vertically in such a column to divide the vapourflow space in such a column into two parallel and separate verticallyextending zones; attached to extend from each side of said plate means,a plurality of tray-like liquid/vapour contact means, the contact meanson each side being located at vertically spaced intervals along saidplate means and the locations of the contact means on one side of saidplate means being staggered relative to the locations of the contactmeans on the other side of said plate means; each tray means includingat least one liquid/vapour contact area having to one side thereof anoutlet area opening into a downcomer and to the opposite side thereof aninlet area adapted to receive liquid descending through a downcomer fromthe next higher contact means which is located on the other side of saidplate means, the arrangement of inlet area, liquid/vapour contact areaand outlet area being disposed in that order along the contact means ina direction parallel to the plane of the plate means whereby liquid flowfrom the inlet area across the liquid/vapour contact area to the outletarea is generally rectilinear and parallel to said plane; the locationalong the width of the dividing means of said active area of the contactmeans on one side of said plate means corresponding with that of thecorresponding active area of the contact means on the other side of saidplate means and the locations of the inlet and outlet areas of saidactive area of said contact means on said one side of said plate meanscorresponding to the locations of the outlet and inlet areas,respectively, for said corresponding active area of said contact meanson said other side of said plate means.
 2. A structure as claimed inclaim 1, in which the inlet and outlet areas each extend in a directiongenerally transverse to the plane of said plate means.
 3. A structure asclaimed in claim 2, in which the boundaries between the inlet area andthe liquid/vapour contact area and between the liquid/vapour contactarea and the outlet area each extend in a direction generally normal tothat of the plane of said plate means.
 4. A structure as claimed inclaim 2, in which each liquid/vapour contact means includes a pluralityof liquid/vapour contact areas each lying between an inlet area and anoutlet area.
 5. A structure as claimed in claim 4, in which each contactmeans has 2n liquid/vapour contact areas, p inlet areas and q outletareas, where n is a positive integer and where in one of the zones p=nand q=n+1 and in the other of the zones p=n+1 and q=n, and wherein theboundaries between the liquid/vapour contact areas and the inlet areasand between the liquid/vapour contact areas and the outlet areas extendin a direction generally normal to the plane of the axially extendingdivider.
 6. A structure as claimed in claim 5, in which n is 1 or
 2. 7.A structure as claimed in claim 1, including means for collecting liquidfrom each downcomer and supplying it to the inlet area of the next lowertray means in the column, which tray means is on the other side of thedivider plate means from the tray whose outlet opens into saiddowncomer, whereby liquid travelling down the column alternates from onezone to the other as it travels from tray to tray down the column.